As per title, if you bought headshot booster and then buy weakening headshot and hollow point, your damage from headshot booster will not receive bonus damage from the shred effects on the first shot.
hollow point does not seem to proc shred for headhunter/headshot booster at all (another bug in itself), but yeah buying weakening headshot / cripling after headhunter does not give the shred to the headhunter proc